Existing systems have been designed for providing secure user authentication over a network using biometric sensors. In particular, an Online Secure Transaction Plugin (OSTP) protocol developed by the Fast Identify Online (FIDO) alliance enables strong authentication (e.g., protection against identity theft and phishing), secure transactions (e.g., protection against “malware in the browser” and “man in the middle” attacks for transactions), and enrollment/management of client authentication tokens (e.g., fingerprint readers, facial recognition devices, smartcards, trusted platform modules, etc.). Details of the existing OSTP protocol can be found, for example, in U.S. Patent Application No. 2011/0082801 (“801 Application”), and the document entitled OSTP Framework (Mar. 23, 2011), both of which describe a framework for user registration and authentication on a network.